1. Field of the Invention
The present invention relates to an improvement in a CFRP-forming base material, or more specifically, to: a stitched carbon fiber base material which exhibits excellent form stability and shapability, ensures excellent handleability in processing a wet prepreg in CFRP manufacturing steps, and facilitates production of a three-dimensional shape; and a wet prepreg using the same.
2. Description of the Related Art
Carbon-fiber-reinforced plastics (CFRP) adopting carbon fibers as reinforcing fibers have light-weight, high-strength, and high-elastic-modulus characteristics, and are therefore used in a wide range of fields including sporting and leisure goods, and aerospace as well as other general industries. In particular, the CFRP are frequently used in aircraft where the weight-saving effect is prominent. Nowadays, the CFRP are also being employed in primary structural materials for the aircraft.
As proposed in Japanese Patent Application Publication No. 2005-14600 (PTL 1), for example, there is disclosed a conventional method of molding a CFRP for aircraft which includes: impregnating a sheet material in which carbon fibers are arrayed in one direction, or a fabric material in which carbon fibers are arrayed in two directions, with a matrix resin to produce a prepreg base material; and layering sheets of the obtained prepreg base material on a molding die, followed by heating and curing in an autoclave.
In the unidirectional prepreg base materials or the fabric prepreg base materials described above, the carbon fibers are arrayed straight, and the base materials can be surely impregnated with the resin. Accordingly, such prepreg materials can fully exert the high strength and the high elastic modulus attributed to the carbon fibers.
However, the unidirectional prepreg base materials and the bidirectional fabric prepreg base materials as described above require operations to cut the base material into appropriate sizes and then to layer them while changing orientations of the cutout pieces, which cause problems of increases in operating burdens and costs.
Further, Japanese Patent Application Publication No. 63-159576 (PTL 2) proposes a method of manufacturing a biased base material with carbon fibers arrayed in directions of ±45°, in which the biased base material is obtained by helically cutting a tubular woven fabric that is woven tubularly.
However, the biased base material manufactured by this method is prone to be elongated in a lengthwise direction. Accordingly, the base material causes a problem when supplied to prepreg processing that the base material is elongated in the lengthwise direction and the array of the carbon fibers therein is considerably disordered.
Furthermore, U.S. Pat. No. 4,484,459 (PTL 3) discloses a method of manufacturing a multiaxial stitched base material. This method is expected to achieve labor saving in layering work, and is actively considered for its application to the CFRP.
Here, using this multiaxial stitched technology, a stitched base material including carbon fibers arrayed in directions of ±45° can be obtained by: layering a sheet including carbon fibers arrayed in parallel in a direction of +45° and a sheet including carbon fibers arrayed in parallel in a direction of −45°; and uniting the sheets together with stitching yarns.
However, in the method of PTL 3 to array the carbon fiber lines, each sheet is formed by feeding carbon fibers shuttlewise between two chains provided behind a stitching m/c (machine). As a consequence, each of the carbon fiber lines is freely movable between the two chains. This leads to a problem that gaps are likely to be formed between the carbon fiber lines.
Accordingly, the stitched base material with the ±45° arrays needs to be formed with a relatively high fiber areal weight. Such a base material has a problem when supplied to the prepreg processing that the inside of the base material cannot be impregnated with the resin due to high viscosity of the resin.
Meanwhile, most of CFRP-molded products such as aircraft components have curved shapes. On the other hand, a prepreg made of the above-described stitched base material with the ±45° arrays is not stretched at all in the lengthwise direction by the presence of the stitching yarns. As a consequence, the prepreg does not possess shapability, so that the prepreg cannot fit properly into a curved surface of a molded product, or may get wrinkled if forced to fit into the curved surface.
The inventors of the present invention have investigated a cause of poor shapability of the base material in the prepreg of this conventional type which makes the prepreg incapable of fitting into an intricately curved surface, and have found out the fact that while the base material is stretchable in its width direction, an extremely low stretch ratio of the stitching yarns in its weaving advancing direction blocks a deformation of the base material.
On the other hand, the inventors have also found out that a cause for the disorder of the carbon fiber lines is that the stitching yarns, if having an increased stretch ratio, are elongated in wet prepreg processing of the carbon fiber lines.